Can gramicidin ion channel affect the dipole potential of neighboring phospholipid headgroups?

Abstract

The cyclic voltammetry behavior of a mercury-supported tethered bilayer lipid membrane (tBLM) incorporating gramicidin A was investigated in aqueous 0.1M KCl at pH6.8, 5.4 and 3. The distal leaflet of the lipid bilayer consisted of dioleoylphosphatidylcholine (DOPC), dioleoylphosphatidylserine (DOPS), dioleoylphosphatidic acid or a DOPC/cholesterol mixture. In passing from pH6.8 to pH3, the midpoint potential between the negative current peak, due to K+ inflow into the spacer, and the positive current peak, due to K+ ejection into the aqueous solution, shifts toward more positive potentials, while the separation between these two peaks decreases. This behavior is interpreted quantitatively on the basis of a model relying on tBLM structural features estimated independently in previous works. The only adjustable parameter is the rate constant for cation translocation across a potential energy barrier located in the hydrocarbon tail region. The behavior is ascribed to a dragging of the lipid headgroups adjacent to the gramicidin channel mouth toward the hydrocarbon tail region, with a resulting decrease in the negative charge of the DOPC phosphate group, or of the DOPS carboxyl group, with decreasing pH.